I dunno but im just using the cotton wool plugs and theyre a bugger to get out once theyve compacted... even after a walk to the lorry over concrete, up the lorry and onto a grass field is enough to set them like concrete!!!
I use cigarette filters soaked in WD40 pushed into the holes with a bradel (99p from Wilkinsons and doesnt bend like farriers nails). Stay in a treat and come out easily enough too.
My farrier gave me some neoprene plugs that are sold in sheets & you press them out & plug the stud holes with them. I tried them at a show & they are really good, easly to put in, easly to pick out, will certainly getting some more when he comes back again.